The knee is a hinge joint, situated between the thigh bone (femur) and shin bones (tibia and fibula). The end of the femur rests in the shallow cup of the tibia, cushioned by a thick layer of cartilage. At the front of the knee joint, the kneecap or patella sits in a groove at the lower end of the femur. The joint is further bolstered on each side by additional cartilages, which sit in between the knee joint. The bones are held in place by tough bands of connective tissue called ligaments. The entire joint is enclosed inside a tough capsule lined with a membrane and filled with lubricating synovial fluid. Extra capsules of fluid, known as bursae, offer extra cushioning. Contraction of the muscles on the front of the thigh (quadriceps) straightens the leg, while contraction of the muscles on the back of the thigh (the hamstrings) allows the leg to bend at the knee.

Topics covered in this section:

• Ligament injuries
• Tendon injuries
• Cartilage injuries
• Patellofemoral Pain
• Osteoarthritis
• What to do if you have an acute knee injury

Ligament sprains

The joint is held together by tough bands of connective tissue called ligaments. These are the medial (inner), lateral (outer), and cruciates (deep). Sudden twists or excessive force on the knee joint, or in some cases repeated microtrauma, can stretch ligaments beyond their capacity. Torn cruciate ligaments bleed into the knee, and typically cause swelling, pain and joint laxity. The anterior cruciate ligament (ACL) situated in the centre of the joint is the knee ligament most commonly injured. A ruptured ACL does not heal by itself and generally requires reconstructive surgery, although it does depend upon your circumstances. Sprained or torn medial (common) or lateral (less common) ligaments cause pain and variable swelling and a feeling of instability.

Tendon injuries

Muscles are anchored to the joints with tendons. A common cause of knee pain is tendon injury. This can occur at any site and if not diagnosed and managed early can result in chronic pain and disability.

Cartilage tears

The knee joint is bolstered on both sides by additional strips of cartilage, called ‘menisci’ or semilunar cartilages. One of the most common knee injuries is a torn or split meniscus. Severe impact or twisting, especially during weight bearing exercise, can tear this cartilage. Tears of the meniscus can also occur in older people due to wear and tear. Symptoms include swelling, pain and the inability to straighten the leg.

Patellofemoral Pain

Although it is commonly thought that most knee pain comes form the ‘proper’ knee joint – ie the joint between the thigh bone and the tibia, this is not true. The 2^nd knee joint – the kneecap or patellofemoral joint – is the joint between the kneecap and the thigh bone and it is the commonest source of pain around the knee. The two main reasons that pain arises from this site are (a) arthritis and (b) patellofemoral pain syndrome, which is pain behind or around the kneecap (patella), resulting from physical and biochemical changes in the kneecap (patellofemoral) joint. Although you may hear it also called ‘chondromalacia’, this is not the case. Chondromalacia is actual fraying and damage to the underlying patellar cartilage. People with patellofemoral pain syndrome have anterior knee pain that typically occurs with activity and often worsens when they are descending steps or hills, or after prolonged sitting. One or both knees can be affected.

NOTE that much of what is said here about patellofemoral pain syndrome also applies to osteoarthritis behind the kneecap.

Causes of Patellofemoral Syndrome & Pain

The patella moves within patellofemoral groove in the femur. Several forces act on the patella to provide stability and keep it tracking properly (see figure).

The patella moves not only in an up-and-down direction, but it also tilts and rotates, so there are various points of contact between the undersurface of the patella and the femur. Repetitive contact at any of these areas, sometimes combined with maltracking of the patella, is the likely mechanism of patellofemoral pain syndrome and also a cause of osteoarthritis in the joint. The result is the classic presentation of pain behind and around the kneecap.

Many theories have been proposed to explain the causes of patellofemoral pain. These include biomechanical, muscular and overuse theories. In general, the cause of patellofemoral pain syndrome is multifactorial.

Overuse and Overload

Because bending the knee increases the pressure between the patella and its various points of contact with the femur, patellofemoral pain syndrome is often classified as an overuse injury. However, a more appropriate term may be "overload," because the syndrome can also affect inactive patients. Repeated weight-bearing impact may be a contributing factor, particularly in runners. Steps, hills and uneven surfaces tend to exacerbate patellofemoral pain. Once the syndrome has developed, even prolonged sitting can be painful (cinema goers knee) because of the extra pressure between the patella and the femur during knee flexion.

Biomechanical Problems and Muscular Dysfunction

No single biomechanical factor has been identified as a primary cause of patellofemoral pain, although many have been hypothesized.

Flat feet or over pronation can upset the tracking of the knee cap. For this reason, orthotics may be useful. 

A high-arched foot provides less cushioning for the leg when it strikes the ground. This places more stress on the patellofemoral mechanism, particularly when a person is running. Proper footwear, such as running shoes with extra cushioning and an arch support, can be helpful.